1. Field of the Invention
The present invention generally relates to a security sensor of a type utilizing a passive-type infrared sensor element and, more particularly, to the security sensor of a type having a disturbance detecting capability for detecting the presence or absence of an obstacle such as, for example, a sticker or label affixed to the sensor casing to disable the security sensor.
2. Description of the Prior Art
An intruder detecting system utilizing the security sensor of the type referred to above is so designed and so configured as to detect an intruder within a detection area or a detection area in reference to the difference between the temperature of a human body and the ambient temperature when the passive-type infrared sensor element receives far infrared rays of light emitted from the human body within the detection area.
It has often been experienced that the intruder detecting system is tampered with an obstacle such as, for example, a transparent sticker or label of a kind capable of transmitting therethrough rays of light ranging from a visible wavelength region to a near infrared wavelength region, but intercepting far infrared rays of light, so that the intruder detecting system may be fooled enough to allow an intruder to trespass on the detection area monitored by the passive-type infrared sensor element. For example, while the intruder detecting system is held inoperative because the detection area is crowded with people moving in and out of the detection area, a potential intruder may enter the detection area and then affixes the obstacle to a light receiving enclosure or an incident side enclosure such as, for example, a sensor lens or cover through which the far infrared rays of light enter, so that the potential intruder can enter again the detection area later while the intruder detecting system is switched in operation with the detection area no longer crowded with people.
In view of the above, the security sensor equipped with a disturbance detector for detecting the presence or absence of the obstacle has been well known in the art and is disclosed in, for example, the Japanese Laid-open Patent Publication No. 2-287278. According to this publication, the disturbance detector used in the security sensor includes a light projecting element and a light receiving element and is so configured that while an obstacle detecting light emitted from the light projecting element is projected towards an inner surface of a lens, which forms a part of the light receiving enclosure of the security sensor and through which far infrared rays of light emitted from a human body pass onto a far infrared sensor element, the light receiving element may receive the obstacle detecting light reflected from the inner surface of the lens. In this structure, in the event that the obstacle is affixed to an outer surface of the lens, the obstacle detecting light reflected form the inner surface of the lens and traveling towards the light receiving element apparently contains a component of light reflected from the obstacle and, therefore, the amount of light incident on the light receiving element is higher when the obstacle is affixed to the outer surface of the lens than that when no obstacle is affixed thereto. By detecting an increase in amount of the light incident on the light receiving element relative to the standard amount of light normally received by the same light receiving element, the disturbance detector can detect the presence of the obstacle on the outer surface of the lens.
It has, however, been found that with the disturbance detector used in the prior art security sensor, detection of the increment of the light reflected from the obstacle is difficult to achieve where the amount of the obstacle detecting light reflected from the obstacle is insufficiently small relative to the standard amount of the light incident on the light receiving element because the obstacle detecting light reflected from the inner surface of the lens may travel astray.
In particular, in the event that the obstacle such as, for example, a transparent sticker of a kind capable of intercepting far infrared rays of light, but transmitting therethrough the obstacle detecting light of a wavelength ranging from a near infrared wavelength region to a visible wavelength region is tightly affixed to a front surface of the lens, the lens and the transparent sticker are integrated together and, hence, the amount of light reflected from the obstacle decreases so extremely that the disturbance detector may fail to detect it. Moreover, since the transparent sticker is virtually indiscernilde with eyes, the presence or absence of the obstacle on the lens is not easy to detect with eyes.
In order to detect the presence of the obstacle such as the transparent sticker of the kind discussed above, attempts have hitherto been made to capture an instantaneous change of the amount of the obstacle detecting light when the obstacle is affixed (i.e., to detect the act of affixing the obstacle) or to employ an increased emitting and receiving power of the disturbance detector. However, the former does not only require the disturbance detector to be activated at all times, but also is susceptible to an erroneous detection resulting from an erroneous operation of the disturbance detector. On the other hand, the latter may often result in an erroneous detection even when small insects traverse.
Accordingly, the present invention has been devised to substantially alleviate the foregoing problems inherent in the prior art security sensors and is intended to provide an improved security sensor having a disturbance detecting capability capable of easily detecting the presence of an obstacle such as, for example, a transparent sticker or label when the latter is affixed to a front surface of the light receiving enclosure of the security sensor.
In order to accomplish the foregoing object of the present invention, there is provided a security sensor having a disturbance detecting capability which includes a carrier body having an infrared sensor element; an incident side enclosure mounted on the carrier body, said incident side enclosure comprising a lens that defines at least one detection area for the infrared sensor element or a cover that covers an incident surface area of the infrared sensor element; a light projecting element for projecting a disturbance detecting beam from inside of the incident side enclosure towards the incident side enclosure; a light receiving element for receiving at least a portion of the disturbance detecting beam from the incident side enclosure; and a detecting circuit for detecting a presence or absence of an obstacle, applied to the incident side enclosure, based on an amount of light received by the light receiving element. A multiplicity of projections are formed on an outer surface of the incident side enclosure so as to define a multiplicity of gaps between the obstacle and the outer surface of the incident side enclosure, when the obstacle is applied to the outer surface of the incident side enclosure.
According to the present invention, even though the obstacle such as, for example, the transparent sticker of a kind capable of intercepting the far infrared light, but allowing the disturbance detecting beam to pass therethrough is applied to the outer surface of the incident side enclosure, the presence of the projections on the outer surface of the incident side enclosure does not allow the transparent obstacle to tightly adhere to the outer surface of the incident side enclosure, leaving gaps between the outer surface of the incident side enclosure and the obstacle. Accordingly, the amount of the disturbance detecting beam reflected from the inner surface of the obstacle increases and, hence, the amount of the light incident on the light receiving element increases correspondingly. The detecting circuit assuredly detects, by detecting the increase of the amount of the reflected light, the presence of the obstacle. Also, even though a small obstacle such as a fly or an insect perches temporarily on the outer surface of the incident side enclosure, and since the amount of the light reflected from such small obstacle is small, there is no possibility of the security sensor functioning erroneously.
In a preferred embodiment of the present invention, the use is made of a light guide member for guiding the disturbance detecting beam reflected from the obstacle towards the light receiving element. Since this light guide member guides the disturbance detecting beams, which has been reflected from the obstacle, so as to travel towards the light receiving element, the freedom of positioning the light receiving element can increase.
Also, preferably, the light guide member is positioned at a location offset from an incident area aligned with the infrared sensor element. Positioning of the light guide member in this manner will not cause the presence of the light guide member to reduce the disturbance detecting capability of the security sensor.
In a preferred embodiment of the present invention, the infrared sensor element and the light projecting elements are mounted on a printed circuit board. This enables a wiring circuit to be simplified.
Also, the incident side enclosure may include a lens having an inner surface formed with a Fresnel lens having rugged lens elements each having a step, in which case the projections are positioned on an outer surface of the lens at respective locations aligned with the steps of the rugged lens elements of the Fresnel lens. According to this design, although the steps correspond in position to a space between the neighboring detection areas defined by the lens elements, the presence of the projections at such portion will not distort the detection areas.
Again in a preferred embodiment of the present invention, the carrier body includes a base for supporting the infrared sensor element, the light projecting element and the light receiving element mounted thereon, and a casing fitted to the base; wherein a lens which is the incident side enclosure is fitted to the casing. The carrier body and the lens cooperate to enclose the infrared sensor element, the light projecting element and the light receiving element while the projections are formed on a center portion of the lens. According to this design, since the projections are positioned where the obstacle is likely to be applied, any act of fooling or tampering with the lens of the security sensor can effectively be prevented.
Yet, the carrier body may alternatively include a base for supporting the infrared sensor element, the light projecting element and the light receiving element mounted thereon, and a cover which is the incident side enclosure and which is fitted to the base so as to enclose the infrared sensor element, the light projecting element and the light receiving element, and wherein the projections are formed on a portion of the cover encompassed in and within the detection area. According to this design, any act of fooling or tampering with the cover of the security sensor with no lens can effectively be prevented.